Mayoral candidate Mike McGinn has throttled back his rhetoric against the replacing the Alaskan Way Viaduct with a deep-bored tunnel, saying if elected, he would honor Monday's unanimous decision by the City Council to move forward with the project.

McGinn said he remains convinced that the $4.2 billion tunnel project is a "terrible decision" for Seattle and that he would continue to question the project's costs and whether Seattle taxpayers could end up paying for overruns. But, he said, it would not be the mayor's place to withhold the city's cooperation in executing the agreement.

"This doesn't change my opposition to the tunnel, it's just my recognition of what the mayor's job is," he said Monday during a meeting with reporters outside City Hall.

Just minutes earlier, the City Council voted 9-0 to codify a mostly-symbolic Memorandum of Agreement with the state that says the deep bore tunnel is a preferred option for replacing the aging, earthquake-damaged viaduct. The agreement also outlines $800 million in construction related to the project that Seattle would cover, including construction of a new seawall, new utility lines, and a waterfront park along a new surface Alaskan Way.

McGinn, a lawyer and environmentalist, has made the tunnel a central focus of his campaign, vowing that he would fight to stop it. He has said he would prefer a less expensive surface Alaskan Way boulevard with transit improvements and increased capacity added to Interstate 5. He said he thinks the questions about costs still will need to be answered before the project moves forward. He said he disagreed with the timing of Monday's vote.

The Memorandum of Agreement marks the fourth in a series of contracts for the project. In September, the Council approved three contracts with the state that authorized more than $480 million in state funds to be used for reconfiguring the south portion of the Viaduct. The tunnel would open in 2015, with related construction wrapping up in 2018.

"The viaduct replacement will provide safe, effective transportation solutions that our region desperately needs," said Council President Richard Conlin.

The city, state and King County had previously struck a deal to move forward with the project, the total cost of which is $4.2 billion. The tunnel itself, with the north and south portals, is estimated to cost $1.9 billion. An environmental review is expected to be completed by early 2011.

Businessman Joe Mallahan, who is competing against McGinn to replace Mayor Greg Nickels, is behind the tunnel project, saying that it is time to move forward after eight years of debate.

Mallahan, who won over much of Nickels' prior support from business and labor because of his pro-tunnel stance, said he was pleased with the vote. He accused McGinn of flip-flopping.

"My opponent has spent the last eight months campaigning on one issue -- stopping the tunnel and our economy from moving forward," he said in a statement. "Now he's changing his position because he's seen the poll numbers and is fighting for his political life. His flip-flopping clearly demonstrates that voters have a choice between a political opportunist or a principled leader and effective manager, like myself, to lead this city and our economy forward."

A handful of tunnel opponents spoke out during Monday's council meeting, including activist Elizabeth Campbell, who has filed two lawsuits accusing the state and federal government of violating environmental regulations by moving forward with the project.

McGinn and others are concerned about a provision included by the state Legislature last spring that says Seattle-area property owners who benefit from the project would be responsible for cost overruns. There are doubts, however, over whether the provision is enforceable.

The state Transportation Department says the tunnel estimates include about $400 million in padding for potential risks and overruns.

Councilman Nick Licata said Monday he wanted to amend the city's agreement to "clearly state" that Seattle would not be responsible for cost overruns. However, he said, he couldn't get more support from the council because Gov. Chris Gregoire was not in favor.

Licata said the City Attorney's office assured him that Monday's vote would not bind the city to paying any cost overruns associated with the state's portion of the project. And future agreements on issues such as utilities and right-of-way, he said, would be more substantive.

"This is a policy statement, it is not a contract -- for those who are concerned that this is the last word on the tunnel project," he said.

Councilman Bruce Harrell said he supported the tunnel, but found the timing of Monday's vote suspect, given the tunnel had become a prominent issue in the mayor's race. The election is Nov. 3.

"It seems like we're trying to sell the tunnel," he said.

In a statement, Mayor Nickels said: "After years of debate and discourse, we are finally getting down to removing the dangerous Alaskan Way Viaduct from our waterfront. I commend the City Council for taking this step and supporting the bored tunnel solution."

"We will preserve transportation choices and open our city up to Elliott Bay for the first time in my lifetime," he said.